Having had a brumby instrument apart completely to install a tacho where the clock lived, I can tell you the orange colour on the speedo dial is generated by an orange piece of plastic, that's shaped like a lightbulb surround.
It sits at the base of the speedo mechanisim and orange light is transfered to the cluster via a simple type of optical plastic. If you can get the same shaped piece of plastic from another subaru cluster of a later design it should fit (they are available in a green blue colour).
Good luck and buy a spare instrument cluster those buggers are fragile
TOONGA